In the drilling of oil and gas wells by the rotary method, aqueous drilling liquids, commonly called "muds" are used during drilling operations to facilitate removal of the dislodged formation particles from the borehole back to the surface, to lubricate the bit, to maintain wellbore integrity, and to assist in controlling formation pressure. Drilling muds usually consist of a mixture of clays, chemicals, and water or oil, all of which are carefully formulated for the particular well/formation being drilled. During drilling operations, the mud is circulated down through the drill string, out the drill bit at the lower end of the drill string, and then up through the annular space between the drill string and the borehole to the earth's surface. The mud which is returned to the surface from the wellbore will contain cuttings produced in the drilling process (especially clay) and may also contain various substances that have been added to give it the desired chemical and physical properties. As a result, it is typically treated at the surface to maintain the correct consistency and other make-up characteristics before being recirculated back down into the wellbore.
It is important that excess solids be removed from the drilling mud because accumulation of these solids can increase drilling costs by slowing down the rate of penetration of the drill bit and by creating logging and other problems such as loss circulation and stuck pipe. As described further below, the common solution to the problem of solids build-up has been to set up sometimes elaborate and costly solids control systems to remove the drill solids from the drilling fluid at the surface. The costs of the drilling mud along with the associated maintenance costs can account for a significant portion of the costs of drilling a well.
With conventional solids removal processes, the mud passes through various types of equipment (such as screen shakers, hydrocyclones and centrifuges) to remove unwanted solids. The problem with these devices is that they are limited to the size of particles which can be separated. The drill solids from the wellbore will range from several millimeters to submicron in size (1 micron=1/25,000 inch). The larger solids (particles having diameters greater than about 20 microns) are typically removed by screen shakers and gravitational separation devices such as hydrocyclones and decanting centrifuges. However, many of the ultra-fine particles (particles having diameters in the range of about 4 to about 44 microns) and the majority of the colloidal particles (particles having diameter less than about 3 microns) will continue to circulate and disperse through the system unless special solids-removal equipment or chemical treatment processes are used.
To remove solids below about 20 microns in diameter, coagulants and flocculants can be added to muds prior to separation with a centrifuge. These chemically enhanced centrifugation ("CEC") processes aggregate the fine particles, which allows them to be more easily separated from the fluid by centrifugation. Coagulants that have been used include aluminum, iron, and calcium salts. Flocculants that have been used include copolymers of acrylamide, which can be cationic, nonionic and anionic. The fluid recovered by centrifugation is returned as needed to the mud system and the separated solids are discharged as waste.
One of the problems with CEC processes is that residue chemicals in the returning effluent may have undesirable effects on mud properties (e.g., poor filtration properties due to flocculation of bentonite clay). This type of process also involves sometimes significant chemical and labor costs. Another significant problem is that these processes will produce very wet solids discharges (typically, about nine barrels of drilling fluid are discarded for each barrel of drill solids removed). Drilling rigs in some cases produce up to about two barrels of liquid waste for every foot of hole drilled. Thus, drilling operations can generate large quantities of waste. Due to increasing environmental concerns and escalating waste treatment and disposal costs, there is growing incentive to reduce the volume of drilling wastes. As a result, the effectiveness of a solids control system for drilling operations can be measured not only by the amount of solids removed from the treated mud, but also by the amount of water in the solids that are removed. Ideally, the solids control system will remove all drilled solids from the treated mud and the removed solids will be essentially dry.
Although not used in the current technology covering separation of solids from a drilling mud, acoustic energy at an intensity below cavitation level has been used in techniques to separate a dispersed phase from a fluid: The application of acoustic energy below cavitation level to a fluid containing a dispersed phase has been used in the biomedical field for blood cell agglomeration, industrial aerosol dust removal, and in unit operations for solid/liquid separation. In these instances, acoustic energy applied to the fluid at an intensity below cavitation level has been shown to cause agglomeration of particles (having a neutral surface charge) in the fluid. Once the smaller particles have agglomerated, it is much easier to then separate the larger agglomerations from the fluid. With each of these applications, it is clear that the intensity level of the acoustic energy applied to the fluid must not be at or above cavitation level because cavitation has been shown to cause particles to degrade or break-up into even smaller sizes, rather than to agglomerate.
One of the conditions responsible for the agglomeration effect of acoustic energy applied to a fluid at an intensity below cavitation level is thought to be the resulting existence of a standing wave in the fluid. The suspended denser particles in the fluid migrate towards pressure node positions located along the standing wave. It is thought that secondary forces such as particle oscillation and Bernoulli force are used to bring about particle-particle collisions and eventually particle aggregation at these pressure nodes. The problem with this approach as applied to separation of fine solids from a typical gel or polymer drilling mud is that no agglomeration, and thus no enhanced separation, will occur in the absence of significant quantities of chemical additives: The drilled solids in drilling muds returned from the wellbore will typically have a negative surface charge. As a result, when acoustic energy below cavitation is applied to the fluid, the particle-particle collisions at pressure nodes of the standing wave will not result in particle agglomeration because the particles having like surface charges will repel each other. To ensure agglomeration, chemical coagulants would need to be added prior to separation. As described previously, the same problems associated with addition of chemicals to the drilling fluid will exist: A very wet solids discharge will be obtained, and there may be undesirable effect on the recycled effluent due to residual chemicals.
Acoustic energy above cavitation level has been harnessed and applied in atomization of liquid, bonding of plastic and metal, cleaning (ultrasonic bath), increasing the activity of metal catalysts, and fusing metal particles, as described in "Scaling Up Ultrasonically Enhanced Processes", Senapati, Ultrasonic International 89 Conference Proceedings, p. 236--243. The effects of cavitation arise from the physical processes that create, enlarge and implode gaseous and vaporous cavities in a liquid. Intense ultrasonic waves generate large alternating stresses within a liquid by creating regions of positive and negative pressure. When a cavity reaches a critical size, the cavity implodes, generating intense heat and tremendous pressure. The shock waves created by the implosion of the cavities has been shown to drive small nickel and zinc particles into one another at speeds of more than 500 kilometers per hour; these collisions were so intense and violent that localized melting took place in the metal particles at the points of impact. However, as previously stated, the application of acoustic energy above cavitation level has been avoided in processes for the separation of dispersed particles from a fluid because the effect of the implosions of cavities and resulting shock waves has been to degrade or disperse the particles to even smaller sizes, rather than to agglomerate the particles.
To summarize, chemically enhanced centrifugation processes and/or the application of acoustic energy below cavitation level may be useful in removing fine solids from drilling fluids. However, there is a substantially unfilled need for an improved process for removing ultra-fine and colloidal solids from the drilling fluids without adding any, or any significant quantities of, coagulants and/or flocculants and without a resulting wet discharge. Furthermore, although this discussion has focused on the problems associated with removal of solids from drilling fluids, it is likely that many similar problems will occur in other solid/liquid separation processes, such as those associated with waste water treatment. The various embodiments of the inventive process described below may be useful in handling solid/liquid separation in such other processes other than solid separation from drilling muds.